fix(tvix/eval): thread thunk forcing errors through correctly

With this, if an error occurs while forcing a thunk (which is very
likely) it is threaded through to the top by wrapping it in the
ErrorKind::ThunkForce variant.

We could use this to generate "stacktrace-like" error output if we
wanted, or simply jump through and discard everything except the
innermost error.

Change-Id: I3c1c8708c2f73ae062815adf490ce935b1979da8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6409
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/errors.rs

@@ -48,6 +48,10 @@ pub enum ErrorKind {
     // These are user-generated errors through builtins.
     Throw(String),
     Abort(String),
+
+    // An error occured while forcing a thunk, and needs to be chained
+    // up.
+    ThunkForce(Box<Error>),
 }
 
 #[derive(Clone, Debug)]